US20220410275A1 - High Energy 3-D Printer Employing Continuous Print Path - Google Patents

Abstract

High-throughput printing, possible with multiple electron beams, is facilitated by a continuous powder bed preparation process operating in parallel to apply and pre-sinter the powder along a continuous helical path. The sintered powder may be self-supporting to allow unconstrained expansion in the radial direction when high energy is used for powder fusion.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
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CROSS REFERENCE TO RELATED APPLICATION
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Background of the Invention
• The present invention relates to three-dimensional printers and in particular to printers using powder materials applied in layers and melted in place.
• Three-dimension printers for implementing additive machining may create printed objects by incrementally depositing material first to a print bed then to previously deposited layers in a layer-by-layer fashion. A variety of different 3-D printing technologies exist. Photo polymerization techniques use lasers to polymerize a thin surface of liquid over a print bed, the latter of which is gradually withdrawn beneath the liquid surface as the object is built up. Extrusion techniques use a similar approach but extrude material such as molten plastic from a nozzle in successive layers. Powder bed systems employ a laser or electron beam to sinter or melt particles of a powder bed into a solid structure. After each layer is formed, additional powder is added on top of that layer and the process repeated.
• Metal objects are most frequently constructed by 3-D printers using powder bed techniques with metallic powders fused by a laser beam or electron-beam. In these techniques, the ability to construct high-resolution, large models using 3-D printing is limited by the limited energy transfer rate of the scanning beam. A slow printing speed has a disproportionate effect on larger, high-resolution models where printing volumes scale exponentially.
• U.S. Pat. No. 9,981,312 issued May 29, 2018, and assigned to the same assignee as the present invention and hereby incorporated by reference, describes a powder printing process using a cathode comb of multiple electron beams that may simultaneously print in unison, potentially substantially increasing the speed of large-area electron-beam printing.
SUMMARY OF THE INVENTION
• The present inventors have recognized that speed gains for large area electron beam printing are limited not only by the rate of energy deposition but also by the foundational steps of preparing the print surface, including applying the print powder and sintering the powder to prevent electrostatic scatter. Accordingly, in one embodiment, the present invention combines a high-flux electron beam with powder bed preparation elements operating in parallel at different locations along a continuous track eliminating the need to stop or interrupt the fusing process. The invention may further better accommodate high rates of energy deposition by applying the electron beam to freestanding sintered powder that is unconstrained by radial walls to better handle expansion caused by large heat input without wasteful excess powder margins.
• More specifically, in one embodiment, the invention provides a three-dimensional printer having a print bed for supporting an object to be printed and a powder dispenser movable with respect to the print bed throughout a path projecting to a closed loop on the print bed for applying a layer of powder over the print bed along a path. A sintering energy source is movable with respect to the print bed throughout the path and positioned along the path after the powder dispenser to sinter the powder throughout a height of the layer, and a multi-cathode electron source is movable with respect to the print bed throughout the path and positioned along the path after the sintering energy source and steerable to selectively liquefy only portions of the sintered powder to produce a printed part.
• It is thus a feature of at least one embodiment of the invention to allow each of these necessary processes to be performed without physical interference on different portions of a continuous path. It is yet another feature of at least one embodiment of the invention to accommodate extremely high electron fluxes by providing an integrated powder substrate with full depth sintering.
• The powder dispenser, sintering energy source, and multi-cathode electron source may operate simultaneously to apply a layer of powder, sinter the powder, and liquefy the sintered powder.
• It is thus a feature of at least one embodiment of the invention provides a system that allows independent tuning of the throughput of each print bed processing step thereby eliminating the need for one process to wait for another process's completion.
• The multi-cathode electron source may be positioned to follow the sintering energy source at a proximity and speed to prevent the average temperature of the sintered layer from decreasing more than 25% between the sintering and the fusing.
• It is thus a feature of at least one embodiment of the invention to allow the sintering process to also preheat the material for fusing, further boosting the print speed.
• The sintered layer may be substantially unconstrained at its limits on either side of the path axis during the fusing process.
• It is thus a feature of at least one embodiment of the invention to accommodate high energy flux printing that can produce substantial thermal expansion and distortion if otherwise constrained by walls such as are required to avoid wasteful, loose powder boundaries.
• The print bed may rotate about an axis and translates along the axis with respect to the sintering energy source and multi-cathode electron source and powder dispenser so that the sintering energy source and multi-cathode electron source and powder dispenser pass helically to trace a circular region of the print bed as powder layers are added along a helical path.
• It is thus a feature of at least one embodiment of the invention to provide a mechanically simple continuous path accommodating an arbitrary number of layers for large print objects.
• The three-dimensional printer may further include a traveling form extending along only a portion of the path and providing radially opposed walls receiving powder from the powder dispenser and retaining the powder therein during sintering, the traveling form moving with respect to the sintered powder layer as the print bed rotates about the axis and translates along the axis with respect to the sintering energy source and multi-cathode electron source.
• It is thus a feature of at least one embodiment of the invention to constrain the print ribbon closely to the printed part to conserve powder while also accommodating high thermal expansion associated with high-energy deposition rates.
• The radially opposed walls may be supported to vary separation of the walls as the walls move with respect to the sintered powder layer.
• It is thus a feature of at least one embodiment of the invention to flexibly provide a continuous sintered layer of varying width tailored to the particular printed part.
• The radially opposed walls may include cooling channels for receiving a flowing coolant.
• It is thus a feature of at least one embodiment of the invention to allow sintering to occur close to the constraining walls to reduce wasted powder usage.
• The sintering energy source may provide a flux per area that varies radially across the print layer to increase sintering near the walls.
• It is thus a feature of at least one embodiment of the invention to provide good mechanical integrity to the sintered layer while accommodating part removal by reduced strength sintering near the printed part.
• The sintering energy source provides an area of simultaneous heating of greater than one square centimeter.
• It is thus a feature of at least one embodiment of the invention to provide a sintering process compatible with high printing speeds practical with higher energy deposition rates.
• The sintering energy source may be electrostatically neutral.
• It is thus a feature of at least one embodiment of the invention to increase the sintering speed without incurring the very electrostatic scattering that the sintering is intended to prevent.
• The invention may include an electronic computer executing a program in stored memory to receive a set of three-dimensional models describing printed parts each assigned to an economic value and generating different combinations of the printed parts with different orientations such as may fit within a predefined printed volume. For each iteration, the computer may evaluate an objective function indicating a total value of the predefined printed volume with a particular combination of printed parts and select an iteration and its combination of printed parts and orientations for printing based on the objective function. The selective iteration is then output for printing on a three-dimensional printer of the type described above.
• It is thus a feature one object of the invention to leverage the features of the above described 3-D printer, such that the scaling limitations of 3-D printing are avoided, resulting in a decrease in cost of 3-D printed parts by maximizing utilization of the print volume.
• These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG.1 is a perspective phantom view of an apparatus constructed according to the present invention and providing a continuous helical printing path, the figure showing a removable upper print head that may be moved between different print beds;
• FIG.2 is a top plan fragmentary view of the continuous printing path (rendered as a straight line for clarity) showing the sequential powder preprocessing station and high-energy fusing station positioned at different locations along the path;
• FIG.3 is a side elevational view of the powder preprocessing and fusing stations ofFIG.2;
• FIG.4 is an elevational cross-section perpendicular to the helical printing path showing a traveling form for preparing a powder material for sintering;
• FIG.5 is a cross-section similar toFIG.4 positioned at the sintering station and showing a radially varying sintering energy flux;
• FIG.6 is a perspective view in phantom of a print volume showing a nesting of multiple parts of different values in the print volume as is practical because the parallel processes of the present invention greatly reduces the time penalty for additional printing in a given volume; and
• FIG.7 is a flowchart of the optimization process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
• Referring now toFIG.1, a three-dimensional printer10 suitable for practice of the present invention may include avacuum housing12 having anupper printhead portion14 and lowerprint bed portion16 which may be connected together during a printing process and separated for pre- or post-printing activity. In this respect, the lowerprint bed portion16 may communicate with avacuum pump15 and provides an airtight chamber in which a vacuum may be drawn during printing by apump15 or supplemented bypump38. It will be appreciated that the lowerprint bed portion16 orvacuum housing12 may be substantially smaller than shown inFIG.1, minimizing the time required to pump down to a vacuum sufficient to prevent metal oxidation and allow for the unimpeded traversal of particle beams.
• When theupper printhead portion14 is separated from the lowerprint bed portion16, avacuum separator31 may maintain the vacuum within the lowerprint bed portion16 through a set of seals and gates allowing the necessary communication between theupper printhead portion14 and lowerprint bed portion16 during printing but sealing the lowerprint bed portion16 when theupper printhead portion14 is removed. After separation, theupper printhead portion14 may be transferred to a secondprint bed portion16′ allowing printing to resume and similarlyupper printhead portion14 may be maintained at vacuum during this transfer through a set of seals and gates. During this separation and transfer, the previous lowerprint bed portion16 may be maintained at vacuum to allow cooldown of the printed part without oxidation. Separately, the incoming lowerprint bed portion16′ may have been already pumped down and maintained at vacuum using itsvacuum separator31 andindependent vacuum pump15.
• Theprint bed portion16 may provide a horizontally extending, disk-shapedprint bed18 supported within the airtight chamber for rotation about a vertical axis20 (z-axis) and for translation motion vertically along thataxis20 by means of a combination of one or morerotary motors22 andlinear motors24.
• Positioned above theprint bed18 in theupper printhead portion14 near its outer rim are a fusingstation26 andpowder preprocessing station28 as will be described in more detail below, each aligned with respect to a common radius of theprint bed18 to process steps of the printing of parts. As theprint bed18 rotates and translates, the printing process proceeds along a helical path with respect to theprint bed18 adding printed layers over previously printed layers. Generally, the helical path projects to a closedcircular path30 on theprint bed18 upper surface. The fusingstation26 andpowder preprocessing station28 may remain fixed with respect to thevacuum housing12 and theprint bed18 moved; however, it will be appreciated that the opposite approach may also be adopted.
• The fusingstation26 may communicate with asecondary vacuum pump38 for providing a harder vacuum at the fusingstation26 than in the remainder of thevacuum housing12 as will be discussed below.
• Control signals alongcontrol lines23 may be provided to each of therotary motors22,linear motors24, fusingstation26, andpowder preprocessing station28 from apower unit32 communicating withelectronic computer34 to provide controlled power, or from theelectronic computer34 directly. Theelectronic computer34 may include one ormore processors36 and acomputer memory29, the latter holding a storedprogram39 coordinating operation of the various above-described components as will be discussed.
• Referring now toFIGS.2 and3, thepowder preprocessing station28 will generally provide a traveling form comprised of an inner and outer upwardly directedwall40aand40bextending a short way along either side of thepath30 and establishing therebetween aprint ribbon43 defining anupper layer42 ofloose powder44. Generally, the radial width of theprint ribbon43 may be large, having a dimension of more than 10 cm and/or more than 30 cm.
• Theloose powder44 will be deposited between the walls40 from anoutlet slot52 of apowder hopper48 and may be a pure metal powder without binder material that would otherwise lower strength or density of the finished product. Thislayer42 ofloose powder44, for most of the printing process other than the first layer, will be deposited on aprevious layer42′ of fused and sintered powder. Thisprevious layer42′ is created from a previous pass of thepowder preprocessing station28 and fusingstation26 around thepath30.
• As theprint bed18 rotates (to the right as depicted), theloose powder44 is drawn against aleveling blade50 which may be fixed with respect to the walls40 andexcess powder44 conducted throughside outlet slots52 in the walls40 to be recycled. The resultinglayer42 passing under theleveling blade50 may be approximately 50 microns thick and will generally be greater than 20 microns and less than 500 microns thick and, in a limiting case, may be a monolayer equal in height to a powder diameter.
• After leveling, the loose powder of thelayer42 may pass under a sinteringenergy source54 that sinters theloose powder44 into a self-supportingsintered matrix62 over the entire height of thelayer42 and substantially the full width of theribbon43 consistent with reducing unnecessary use ofpowder44. As is understood in the art, sintering does not fully melt thepowder44 but simply temporarily binds the powder grains together at their surface to provide a self-supportingsintered matrix62 that is self-supporting but under mechanical pressure may nevertheless be broken apart at a later stage for recycling of thepowder44. The sinteringenergy source54 may apply a broadly focused scanned beam of light or electrons (having a focal spot of at least 1 mm), but preferably thesintering energy source54 provides a broad area energy transfer, for example, simultaneously heating a full width of theribbon43 using a fixed array of LEDs, laser diode arrays, orlasers60 together capable of simultaneously eliminating areas greater than 1 square cm and typically greater than 30 square cm. Broad area simultaneous heating allows rapid sintering and preheating of thelayer42 consistent with the expected printing speed at the fusingstation26.
• The invention contemplates that a wide variety of different heating mechanisms may be used for thesintering energy source54 including but not limited to radiative infrared heaters and electrical induction heaters. Preferably, the sinteringenergy source54 is electrostatically neutral so that thepowder44 is not electrostatically charged prior to sintering such as would promote its scattering or dispersal.
• It will be appreciated that the treatment area of the sinteringenergy source54 may extend freely along the path30 (limited only by the full distance between thepowder preprocessing station28 and fusing station26) to allow the process of sintering to be easily adjusted to the speed of fusion and to permit the time for heat to spread through theentire layer42 for uniform sintering.
• The resultingsintered matrix62, as noted, is desirably sintered over its entire depth so as to be self-supporting as it moves beyond the walls40 with rotation of theprint bed18. It should be noted that once thelayer42 moves away from the walls40 with rotation of theprint beds18, and as it moves to be received under the fusingstation26, it is substantially unconstrained in the radial direction.
• Referring toFIG.2, desirably the sintering process may also preheat thepowder44 to improve the speed of fusing of thepowder44 by the fusingstation26 and to reduce temperature differences that would result in unnecessary thermal stress. In this regard, the location of the sinteringenergy source54 and speed of rotation of theprint bed18 may be managed so that thepeak temperature65 of thelayer42 upon exiting the sinteringenergy source54 does not decay more than 25% prior to fusing.
• The fusingstation26 may provide for abottom surface70 that closely overlies the upper surface of thelayer42 as it is received under the fusingstation26, a small gap whose size and length provide a high air resistance channel sufficient so thatvacuum pump38, communicating with acavity74 extending upwardly from thatsurface70, may maintain a harder vacuum in that cavity74 (<₁₀4 Torr) than the remainder of the vacuum housing12 (>104 Torr).
• Exposed downwardly within thecavity74 is a set ofelectrodes72 linearly arrayed in acathode comb75 extending radially across theprint ribbon43. Surrounding the array ofelectrodes72 is amagnetic steering coil76 that may be used to direct electron-beam80 over thelayer42 for fusing a portion of that layer into a printedpart82. The fusing process liquefies thepowder44 within the outlines of the printedpart82 in an amount sufficient to create a uniform and essentially void-free solid material surrounded byunprinted sintered matrix62.
• Acathode comb75 suitable for use in the present invention is described in U.S. Pat. No. 9,981,312 referred to above.
• Thevacuum pump38 and the close spacing of thebottom surface70 of the fusingstation26 to the upper surface of thelayer42 allows the introduction of a jet of coolinggas73 out from thelower surface70 to impinge upon thelayer42 to promote the formation of metallic glasses in the molten material created by theelectrode72 without unduly affecting the high vacuum required for the electron-beam80.
• It will be appreciated that with continued rotation of theprint bed18, theupper layer42 becomes one of thelower layers42′ and that this process may be repeated as theprint bed18 is drawn downward to provide a helical path of printedlayer42 of arbitrary height. The fusing process generally joins the material oflayer42 to similarly fused material of anunderlying layer42′. As noted above, during the fusing process, the self-supportingsintered matrix62 is substantially unconstrained on the radial direction so as to better accommodate high thermal expansion at the instant of fusing and shortly thereafter without distortion that might be caused by compression between constraining walls.
• Referring now toFIG.4, as noted, desirably the sintering process oflayer42 will occur over the full radial width of theprint ribbon43 and the full height of thelayer42 and will be sufficient to create sintered connections betweenlayer42 andlayer42′. In order that the walls40 may withstand the close proximity to such high temperatures, the walls40 are desirably constructed of a high temperature material such as a ceramic or high temperature metal alloy, and may include coolingchannels83 for circulating coolant through the walls40 and aheat exchanger88 using apump90 to remove excess heat. The opposed surfaces of thewalls40aand40bmay be arranged to reduce adhesion and thus a tearing ofpowder44 from thelayer42, for example, by surface treatments or mechanical means such as a movable belt surfaces minimizing contact time and temperature and hence the propensity of thepowder44 to fuse with the walls40. In some cases, the walls40 may be grounded to assist in the neutralization of any charge.
• In some embodiments, one or both of thewalls40aand40bmay be mounted on anactuator84 allowing the separation distance of thewalls40aand40bto be adjusted, for example, to accommodate printedparts82 of different widths possible with different widths ofprint ribbon43.Such actuators84 also allow the separation between thewalls40aand40bto be adjusted dynamically with rotation of theprint bed18, for example, to narrow or widen theprint ribbon43 as a function of rotation. Generally, thewalls40aand40bmay also be spring mounted viasprings85 to provide some ability for the walls40 to accommodate uneven surfaces of the print layers42 by moving vertically, radially, or circumferentially.Actuators84 or springs85 may also provide slip sticking motion to thewalls40aand40bwhere they travel with thelayer42 to permit completion of the sintering then snap back periodically to break any adhesion.
• Referring now toFIG.5, the sinteringenergy source54 may provide for varyingheat flux89 in the radial direction to accommodate the slight keystoning of the area of thelayer42 resulting from the circularhelical path30. In addition, the flux perarea91 provided by the sinteringenergy source54 may be adjusted to increase the amount of sintering at the radial edges of thelayer42 to provide improved resistance to powder loss during the printing process and better support of the powder around the printed object.
• Generally, the height of the walls40 is such as to extend only slightly below thecurrent layer42, for example, halfway into the next succeedinglayer42′ to reduce resistance and heat conduction, the sintering serving to provide necessary cohesion in the powder in thelower layers42′.
• Referring now toFIGS.6 and7, the parallel nature of the printing process provided by the above embodiment exacts little or no time penalty for increased printing area (unlike a scanning system) and, accordingly, the present invention may opportunistically operate to surround the printedpart82 withother parts82′ having different economic values or urgency to lower the overall cost of printing per part. This process may be implemented by the computer34 (shown inFIG.1) as indicated byprocess block101 by receiving a set of different printed part models each assigned to a different value. The value may be the economic value of selling the part with a given delivery schedule. Atprocess block102, different arrangements of theparts82 and82′ may be iteratively evaluated using an objective function summing together the total value of the composite print holding thepart82 and one ormore parts82′. In between each iteration, different combinations and orientations ofparts82 and82′ may be implemented with a virtual tumbling of theparts82 and82′ (rotated in 3 directions and translated in 3 directions). Local maxima in the objective function are avoided using stochastic methods such as simulated annealing or the genetic algorithm.
• Atprocess block104, a printing of the collection ofparts82 and82′ may be implemented containing a larger proportion of solid metal so as to also provide better conduction and reduced need for the ancillary support structure described below.
• Generally, it will be understood that additional support for the self-supportingsintered matrix62 around thepart82 may be provided by printing afrangible scaffold94 in the radial outermost edges of thelayers42. Generally, the self-supportingsintered matrix62 has a strength to be self-supporting but to be readily mechanically removed from thepart82 without damage, for example, by high pressure water streams, airstreams, bead blasting, mechanical abrasion, or the like.
• Referring again toFIG.4, the upper surface of theprint bed18 may include a raisedrim100 providing a supporting surface over the width of theribbon43 and allowing a surface grinding operation to be performed after removal of thelayers42 for a fresh printing. Prior to printing, asintered layer42 may be applied to this raisedportion100 so that printing startup may be accelerated.
• Although the highest printing speeds may be achieved with a charged electron beam fusing source, the fusing source may be from other energy sources as is understood in the art, including a high power laser beam, charged particle beam, laser diode array, inductive heater array, infrared heater array, charged particle beam or multiple thereof.
• Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context.
• When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.
• References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network.
• It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties
• To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke 35 U.S.C. 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim.

Claims (22)

1. A three-dimensional printer comprising:

a print bed for supporting an object to be printed;

a powder dispenser movable with respect to the print bed throughout a path projecting to a closed loop on the print bed for applying a layer of powder over the print bed along a path;

a sintering energy source movable with respect to the print bed throughout the path and positioned along the path after the powder dispenser to sinter the powder throughout a height of the layer; and

a multi-cathode electron source movable with respect to the print bed throughout the path and positioned along the path after the sintering energy source and steerable to selectively liquefy only portions of the sintered powder to produce a printed part.

2. The three-dimensional printer ofclaim 1 wherein the powder dispenser, sintering energy source, and multi-cathode electron source may operate simultaneously to apply a layer of powder, sinter the powder, and liquefy the sintered powder.

3. The three-dimensional printer ofclaim 1 wherein the multi-cathode electron source is positioned to follow the sintering energy source at a proximity and speed to prevent an average temperature of the sintered layer from decreasing more than 25% between sintering and liquefying of the powder.

4. The three-dimensional printer ofclaim 1 wherein the sintered layer is substantially unconstrained at its limits on either side of the path axis during the liquefying process.

5. The three-dimensional printer ofclaim 1 wherein in the print bed rotates about an axis and translates along the axis with respect to the sintering energy source and multi-cathode electron source and powder dispenser so that the sintering energy source and multi-cathode electron source and powder dispenser pass helically to trace a circular region of the print bed as powder layers are added along a helical path.

6. The three-dimensional printer ofclaim 5 further including a traveling form extending along only a portion of the path and providing radially opposed walls receiving powder from the powder dispenser and retaining the powder therein during sintering, the traveling form moving with respect to the sintered powder layer as the print bed rotates about the axis and translates along the axis with respect to the sintering energy source and multi-cathode electron source.

7. The three-dimensional printer ofclaim 6 wherein the radially opposed walls are supported to vary separation of the walls as the walls move with respect to the sintered powder layer.

8. The three-dimensional printer ofclaim 6 wherein the radially opposed walls include cooling channels for receiving a flowing coolant.

9. The three-dimensional printer ofclaim 6 wherein the sintering energy source provides a flux per area that varies radially across the print layer to increase sintering near the walls.

10. The three-dimensional printer ofclaim 1 wherein the sintering energy source provides an area of simultaneous heating of greater than one square centimeter.

11. The three-dimensional printer ofclaim 1 wherein the sintering energy source is electrostatically neutral.

12. The three-dimensional printer ofclaim 1 further including:

an upper housing holding at least one of the powder dispenser, sintering energy source, and multi-cathode electron source;

a lower housing releasably interfacing with the upper housing during printing and holding the print bed; and

a vacuum separator maintaining a vacuum in the lower housing during separation of the upper and lower housings.

13. A three-dimension printer comprising:

a print bed for supporting an object to be printed;

a powder dispenser movable along a path for applying layers of powder over the print bed;

a first energy source movable along the path behind the powder dispenser to produce a sintered powder layer;

a traveling form providing two walls opposed across an axis receiving therebetween powder from the powder dispenser and retaining the powder therebetween during sintering; and.

a second energy source positionable to selectively liquefy selected portions of the sintered powder to produce a printed part;

wherein the traveling form moves along the path away from the sintered powder layer after sintering so that the sintered powder layer is substantially unconstrained at its limits on either side of the path during the liquefying process.

14. The three-dimensional printer ofclaim 13 wherein the first energy source, second energy source, powder dispenser, and traveling form are movable throughout a path projecting to a closed loop on the print bed and may operate in parallel to apply a layer of powder, sinter the powder, and liquefy the sintered powder at different locations along the path.

15. The three-dimensional printer ofclaim 13 wherein the second energy source is positioned to follow the first energy source at a proximity and speed to prevent an average temperature of the sintered layer from decreasing more than 25% between the sintering and the liquefying.

16. The three-dimensional printer ofclaim 13 wherein in the print bed rotates about an axis and translates along the axis with respect to the first energy source and second energy source and powder dispenser so that the first energy source and second energy source and powder dispenser pass helically to trace a circular region of the print bed as powder layers are added along a helical path.

17. The three-dimensional printer ofclaim 13 wherein the two walls of the traveling form are supported to vary separation of the walls as the walls move with respect to the sintered powder layer.

18. The three-dimensional printer ofclaim 13 wherein the first and second walls include cooling channels for receiving a flowing coolant and further including a heat exchanger and coolant circulation pump communicating with the cooling channels.

19. The three-dimensional printer ofclaim 13 wherein the first energy source provides a flux per area that varies radially across the print layer to increase sintering near the walls.

20. The three-dimensional printer ofclaim 13 wherein the sintering energy source provides an area of simultaneous heating of greater than one square centimeter.

21. The three-dimensional printer ofclaim 13 further including:

an upper housing holding at least one of the powder dispenser, sintering energy source, and second energy source;

a lower housing releasably interfacing with the upper housing during printing and holding the print bed; and

a vacuum separator maintaining a vacuum in the lower housing during separation of the upper and lower housings.

22. A three-dimension printer system comprising:

an electronic computer executing a program in stored memory to:

(a) receive a set of three-dimensional models describing printed parts each assigned to an economic value;

(b) generate different combinations of the printed parts with different orientations such as may fit within a predefined printed volume;

(c) for each iteration, evaluate an objective function indicating a total value of the predefined printed volume with a particular combination of printed parts; and

(d) select an iteration and its combination of printed parts and orientations for printing based on the objective function; and

a three-dimensional printer receiving the selected iteration from the computer and having:

(a) a print bed for supporting an object to be printed;

(b) a powder dispenser movable with respect to the print bed throughout a path projecting to a closed loop on the print bed for applying a layer of powder over the print bed along a path;

(c) a sintering energy source movable with respect to the print bed throughout the path and positioned along the path after the powder dispenser to sinter the powder throughout a height of the layer; and

(d) a second energy source movable with respect to the print bed throughout the path and positioned along the path after the sintering energy source and steerable to selectively liquefy only portions of the sintered powder to produce the selected iteration.

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